In the manufacturing environment, the traditional method of measuring dimensions is with mechanical gauges. Linear distances are measured with rulers, calipers, metal tape or precision mechanical stages. These methods involve mechanical contact with the surfaces, and can be very slow because of the manual adjustments required. Further, many types of objects such as rotating shafts, spindles and wheels cannot be conveniently measured mechanically while they are in motion.
Electronic alternatives to mechanical gauges are usually based on capacitance or eddy currents. These devices have the advantage of a direct measurement of a dimension without manual comparison with a standard. This direct measurement capability increases the speed of measurement and reduces errors due to distortion when a reference piece is contacted with the test piece. However, electronic gauges can be material dependent, and usually do not work at all with non-metallic materials such as fiberglass, plastics and some kinds of composite materials. Also, their operational range may be very limited.
Because of the recognized limitations of mechanical gauges, alternative optical methods involving time-off flight laser radar are of interest. The following articles discuss various examples of related prior art in laser techniques for distance measurement.
In an article entitled "Laser Radar Range Imaging Sensor For Commercial Applications" by K. G. Wesolowicz and Robert E. Sampson, SPIE Proc. Vol. 783, p. 152 (1987), there is described an imaging laser radar system employing a single frequency (0.72 GHz) intensity modulation of a GaAlAs laser diode operating at 0.82 .mu.m. The target range x is obtained from the phase delay of the modulation. Since the phase has an implicit 2.pi. ambiguity, the range measurement has a corresponding ambiguity interval. For a modulation frequency v of 0.72 GHz, this interval is 8.2 inches. The article claims a resolution for this radar of between 0.032 and 0.004 inches at 3 feet, depending on the type of target, the measurement time, and the application. It is not clear that the absolute accuracy of the instrument is also 0.004 inch, since this would appear to require 40 ppm linearity in the phase measurement.
In an article entitled "Laser-diode Distance Meter in a KERN DKM 3A Theodolite" by A. Greve and W. Harth, Applied Optics, Vol. 23, No. 17, p. 2982 (1984), there is described an intensity-modulated laser radar that uses a phase locking technique to measure the relative phase .phi.. The modulation frequency is in the 1.745-1.8 GHz range. The ambiguity in the range measurement, at least in principle, can be removed by varying the modulation frequency. In Table II of the article, a distance measurement variation of 85 .mu.m at 2.9 m is claimed. FIG. 3 of the article shows large distortions in the measurement curves that imply a much lower absolute accuracy.
In an article entitled "High-Precision Fiber-Optic Position Sensing Using Diode Laser Radar Techniques" by G. Abbas, W. R. Babbitt, M. de la Chapelle, M. Fleshner, J. D. McClure, and E. Vertatschitsch, there is described a linear position sensor with fiber-optic signal distribution. The sensor uses a frequency-chirped, intensity-modulated laser diode. The intensity-modulation bandwidth is 6 GHz. Absolute distance is obtained by determining the beat frequency between the laser modulation and the delayed modulation of the return signal. The beat frequency is found by high-speed digital Fourier transforming of the beat signal. This approach has the important advantage that several sensor heads may be connected by fiber optics to the same source and detection module, provided that the possible variations in range to each of the heads do not overlap. The experimental system described in the article achieves 58 .mu.m RMS range error over 100 cm using a 1 ms chirp duration and a signal processing time of 50 .mu.sec. A resolution of 10 .mu.m is projected for an improved version of this system. Although the achievements and specifications of this instrument are consistent with some of the objectives of the present invention, the system uses a highly cooperative target (a retroreflector) and expensive radio-frequency hardware. Modification of this system for non-cooperative surfaces in manufacturing may not be practical or cost effective.
In an article entitled "Utilizing GaAlAs Laser Diodes as a Source (sic) for Frequency Modulated Continuous Wave (FMCW) Coherent Laser Radars" by A. Slotwinski, F. Goodwin and D. Simonson, SPIE Vol. 1043, p. 245 (1989), there is described an instrument that uses optical interferometry to generate beat signals between local and time-delayed optical frequencies. The frequency modulation is achieved by thermal tuning of a laser diode cavity length. The thermal tuning is easily effected by precisely controlled variation of the laser excitation current and is thus much easier to obtain over large bandwidths (&gt;5 GHz) than an intensity-modulation chirp. The article claims a resolution of 1 mil (25 .mu.m) by using a reference length for continuous calibration. However, high accuracy and reliability can only be obtained with carefully characterized and monitored single-mode laser diodes. The commercial system is also very expensive and may be sensitive to vibration.
Laser interferometers are widely employed for high-precision displacement measurement and very-high resolution surface profilometry. An example of a commercial instrument in use is the well-known Hewlett Packard Laser Gauge. However, a problem with these instruments relates to the interference phase ambiguity. Interferometry with a single, constant wavelength cannot be used to measure a distance without ambiguity of one-half of one wavelength. Thus, the beam cannot be broken and only highly-reflective, "cooperative" targets such as mirrors and retroreflectors can be used. This seriously limits the applicability of interferometry for measurement tasks in manufacturing.
One known method to extend the range of metrology applications for interferometry is to measure the interferometric phase at two or more distinct wavelengths. This is the method that most closely relates to the present invention. The difference between the interferometric phase measurements at two vacuum wavelengths .lambda..sub.1 and .lambda..sub.2 corresponds to a synthetic wavelength .LAMBDA. given by EQU 1/.LAMBDA.=1/.lambda..sub.1 -1/.lambda..sub.2 ( 1)
If the measured interferometric phase is .phi..sub.1 and .phi..sub.2 for the two wavelengths, then the distance x can be measured to within an interval .LAMBDA. by using EQU x=.LAMBDA.(.phi..sub.1 -.phi..sub.2)/4.pi. (2)
In that the synthetic wavelength may be large compared to visible-light wavelengths, it is possible to measure larger distances before phase ambiguities contribute to measurement errors. If several synthetic wavelengths of different size are used, it is possible to make measurements of successively higher precision to "ladder down" to high accuracy without the inconvenience of phase ambiguities. One of the advantages of this approach over the intensity-modulation techniques described above is that the synthetic wavelength can be made very much smaller and the precision proportionally better than is practical with direct-detection techniques.
An article entitled "Absolute distance interferometry, "by N. A. Massie and H. John Caulfield, SPIE Proceedings, Vol. 816, pp. 149-157 (1987) summarizes the prior art for this type of multiple-wavelength laser ranging technology. The basic principles are described, and several implementations described in other journal articles are presented. Most of these examples involve complex and expensive gas or tunable dye lasers for generating multiple wavelengths. In most cases, no more than two wavelengths are obtainable at any one time from the source, thus increasing the complexity by requiring time-multiplexing and automated laser tuning.
In an article entitled "Absolute optical ranging with 200-nm resolution" by C. Williams and H. Wickramasinge, Optics Letters, Vol. 14, pp. 542-544 (1989), there is described a two-wavelength interferometer requiring the use of two independently-controlled and aligned GaAlAs single-mode laser diodes. This system has the advantage that it uses relatively inexpensive and compact laser diodes. However, the data shows a very small demonstrated operational range (less than 1 mm) and a complex and expensive system of acousto-optic modulators for time-multiplexing the signals for the two wavelengths was used. Changing the synthetic wavelength required modifying the operating conditions (changing the temperature and excitation levels) of the lasers.
In an article entitled "Two-wavelength scanning spot interferometer using single-frequency diode laser" by A. J. den Boef, Appl. Opt., Vol. 27, pp. 306-311 (1988), there is described the use of two single-frequency laser diodes operating simultaneously, with the wavelength separation achieved by polarization. Only one synthetic wavelength is available at a given time.
In an article entitled "Laser diode technologies for in-process metrology," by P. de Groot, SPIE Proc., Vol. 1333, Paper 21 (San Diego, July, 1990), there is described a two-wavelength interferometer using a single, multiple-wavelength laser diode. The laser used was a conventional two-wall Fabry-Perot device exhibiting multiple longitudinal oscillation modes over a spectral width of about 1.2 nm. The interferometric phase for each of the wavelengths was detected with a wavelength-selective detection system involving a diffraction grating. Only one synthetic wavelength (700 .mu.m) was used because of the limited spectral width of the source. Further, the multimode diode had poor temporal coherence and could not be used for interferometry for optical path lengths exceeding 1 mm.
In an article entitled, "Interferometric displacement sensing by visibility modulation," by T. A. Berkoff and A. D. Kersey, OFS '89, 78-82 (Springer Verlag, 1989), there is described a fiber interferometer in which an integrated-optic intensity modulator is used to generate a carrier-suppressed, synthesized two-wavelength source with frequency separation between 10 and 100 MHz. Two kinds of measurements are described. One is the determination of distance by measuring the frequencies for which the fringe visibility is nulled. The other is displacement sensing by locking onto one of these fringe-visibility minima. Since the distance measurement method described in this article requires a variation frequency separation (90 MHz) that is comparable to the largest attainable separation (100 MHz), there is little advantage of this approach over the FMCW method described by Slotwinski et al. cited herein. Also, because the 100 MHz frequency separation of the synthesized modes is relatively small, the resulting resolution is only .+-.2 mm.
The above cited prior art does not adequately meet the simultaneous requirements of accuracy, operational range, cost and low power for the measurement applications of interest to this disclosure.